1. Field of the Invention
The present invention relates generally to pressure vessels, and more particularly, to pressure vessels used for storage of cryogenic and other materials in rocket launch and space vehicle applications.
2. Description of the Related Art
In aerospace applications, pressurized propellant tanks may be fabricated by filament winding fiber reinforcement over a thin walled metallic liner. Carbon or fiberglass fibers provide the required strength without the weight penalty associated with an all-metallic tank. Unfortunately, composite pressure vessels with metallic liners present a thermal stress problem, when used to store cryogenic materials. Specifically, significant differences in the coefficients of thermal expansion (CTE), between the metallic liner and the composite outer shell, result in high thermal stresses at the interface. These thermal stresses can be significant enough to cause rupture of the vessel if not addressed. A vessel fabricated with only composite materials would obviate the disadvantages of using a metallic liner. In lieu of a metallic liner, a composite shell can be used and reinforcement fiber wound over it.
There are basically two techniques for a fabricating a composite shell to filament wind over, (1) hand layup and (2) filament winding. In the hand layup process, sections of the material in the form of fabric are laid over a tool (or pattern) that defines the internal surface of the vessel or sections of the vessel. The fabric, for example, may be fiberglass or graphite fabric. The resulting composite (or laminate) consists of layers of fabric impregnated with a matrix binder, such as an epoxy resin. The resin is applied wet and is cured to a hard shell.
After curing, the tool must be removed, which requires that the part of the vessel so formed must have an open end through which the tool may be withdrawn or the tool can be fabricated from a material such as eutectic salt which can be dissolved. The simplest and most direct approach for removing the tooling is to fabricate the composite shell in two halves, which are later joined together with a splice (or “bellyband”) of similar composite material.
The filament winding technique for fabricating a composite shell is similar, except the material takes the form of continuous bands of fiberglass or graphite fibers, either previously impregnated with a matrix material or impregnated during winding. The fibers are filament wound over a rotating and removable form. For the filament winding process, the vessel is prepared as a whole vessel, which must be cut in two to remove the tool.
Composite pressure vessels for aerospace applications can be very expensive due largely to the need for an autoclave. Autoclaves control the temperature and pressure during curing and can be expensive devices, especially if the vessels to be manufactured are large.
It will be appreciated from the foregoing that there is a need for an all-composite pressure vessel that has no need of a metal liner, and preferably has no need for autoclaving during fabrication. The present invention satisfies this need.